The present description relates to an automatic transmission for automotive vehicles, particularly relates to an arrangement of gear-train of an automatic transmission.
There have been proposed various arrangements of a plurality of planetary gear sets to achieve six or more forward speed ratios. As is well known in the art, a planetary gear set consists of three rotational elements that are a sun gear, a ring gear, and a planetary carrier rotationally supporting planetary pinions, each of which engages with both the sun gear and ring gear. When a single planetary gear set is made up with only these three basic rotational elements comprising a single sun gear, a single ring gear and a single carrier, it is so called a single pinion type.
In addition to the single pinion type, there are also proposed various types of composite planetary gear sets for creating more than three rotational elements within a single planetary gear set. One example of the composite planetary gear sets is shown in U.S. Pat. No. 6,884,197, which presents a six-speed automatic transmission comprising three planetary gear sets. One of the three planetary gear sets has two sun gears, a single carrier, and a single ring gear, thereby creating four rotational elements from the one single planetary gear set.
In the '197 patent, all of the three planetary gear sets are arranged coaxially on a single input shaft. Also, an output gear is arranged coaxially on the input shaft. Therefore, a minimum number of shafts are required, so that overall size of the automatic transmission may be reduced advantageously for mounting it on an automotive vehicle, especially when the engine and the transmission are mounted laterally on the vehicle.
However, the composite planetary gear set presented in the '197 patent may have a disadvantage in noise and vibration, because the pinions may be longer for engaging with the two sun gears, and may have more tendency to tilt than the single pinion type.
Further in the '197 patent, the input shaft transmits torque to the single carrier of the planetary gear set having the two sun gears through a clutch and a connecting member arranged between the two sun gears. Because each of the sun gears needs to be connected to a frictional member or a connecting member, the clutch is forced to be located offset from the planetary gear set in the axial direction. That may cause the input shaft longer, and eventually cause the overall size of the automatic transmission greater.
Therefore, there is a need to improve an automatic transmission of the type presented in the prior art in terms of the noise or vibration and the size.